This invention relates to a shading correction device performing shading correction of an original reader.
Generally, a conventional original reader in which a solid-state image pickup device such as a charge coupled device (CCD) is employed as an image sensor is configured as illustrated in FIG. 1 in which an image sensor 1 performs photoelectric conversion of the image information presented on an original MS. The image information irradiates the surface of the image sensor 1 in the form of reflected light as a result of irradiating the original MS through an original supporting glass PG by a light source LP such as a fluorescent lamp to produce a picture signal CE. This picture signal CE is then applied to a binary conversion circuit 200 via an amplifier 2. At the binary conversion circuit 200, a binary converted signal BS is formed based on the comparison of the applied picture signal CE with a binary conversion reference level E.
However, in such conventional original reader the level of the picture signal produced by the photoelectric conversion becomes uneven due to the following reasons, resulting in unfavorable effects on the signal processing at the binary conversion circuit 200.
(a) Light from the light source LP has an uneven luminance distribution and the luminance typically decreases with the positional change from the center to the end of the light source. PA1 (b) Light through the periphery of the lens LS is dark according to the cos fourth power law, and the level of the reflected light falls at around the periphery. PA1 (c) There is a variation in the sensitivity of the solid-state image pickup device to be used as the image sensor 1.
FIG. 2 shows the descrepancy between the image information on the original and the binary converted signal BS.
That is, though the image information on the original MS shall be picked up in one scan precisely as shown in FIG. 2(a), due to the reason given in (a) through (c) above the picture signal CE after the photoelectric conversion becomes uneven as shown in FIG. 2(b), and when the signal is binary converted based on the comparison with the binary conversion reference level E at the binary conversion circuit 200 the resulted binary converted signal BS becomes far different from the actual image information of the original MS as shown in FIG. 2(c). Thus, the original MS cannot be reproduced with fidelity when the image is reproduced based on such binary converted signal.